%AFiske, P.S.
%AHargaves, R.B.
%D1994%K58 GEOSCIENCES; METEORITES; IMPACT SHOCK; CRATERS; STRATIGRAPHY; GRAVITY SURVEYS; MAGNETIC SURVEYS; MONTANA; IDAHO
%MOSTI ID: 86889; Legacy ID: DE95014513
%PMedium: ED; Size: 18 p.
%TThe Beaverhead impact structure, SW Montana and Idaho: Implications for the regional geology of the western U.S.
%Uhttp://www.osti.gov/scitech//servlets/purl/86889-ZkvWTF/webviewable/
%XThe Beaverhead impact structure in SW Montana and Idaho is an allochthonous fragment of a large impact structure ({approximately} 100 km diameter) that was transported some distance eastward during the Cretaceous Sevier orogeny. It is the first tectonic fragment of a large impact structure identified in the geologic record. The present evidence for impact consists of shatter cones, pseudotachylites, and planar deformation features in quartz. The age of the impact is not well constrained but is estimated to be Neoproterozoic to Cambrian (1000-500 Ma). The Beaverhead impact event must have created other features that may be preserved, elsewhere in western Montana and Idaho. These include proximal and distal ejecta (which may be misinterpreted as diamictites and/or tuff horizons) and other fragments of the crater floor containing shatter cones and pseudotachylite. A large circular gravity, magnetic and topographic anomaly, which could be the root of the impact structure, has been identified near Challis, Idaho. An enigmatic lithic tuff, identified in drill cores from the Challis area and an intraformational quartzite breccia in the Leaton Gulch area may be impact-related deposits, but no definitive evidence of shock metamorphism has been observed in these materials. The discovery of more pieces of the Beaverhead puzzle, as well as the recognition of other large impacts in the geologic record, are likely once the regional geologic community grows to accept the incidence of such events and becomes more familiar with the features of shock metamorphism in the field. To that end, the community of geologists in this area should integrate the Beaverhead structure into their research and teaching curriculum.
%0Conference
%@UCRL-JC--116524; CONF-9308269--1; Other: ON: DE95014513; CNN: Grant EAR 92-05975; TRN: 95:005542
United StatesOther: ON: DE95014513; CNN: Grant EAR 92-05975; TRN: 95:005542Wed Nov 04 08:33:25 EST 2009OSTI as DE95014513LLNL; SCA: 580000; PA: EDB-95:114720; NTS-95:021351; SN: 95001412127English